US3897010A - Method of and apparatus for the milling of granular materials - Google Patents

Abstract

In a fluid-energy or jet-milling system in which a gas stream entrains a granular or particulate material in a jet against a surface to further comminute or pulverize the milled material, the operating or carrier gas is compressed prior to introduction into the mill, expands as a jet in the mill chamber and is separated from the comminuted solid matter thereafter. The gas stream is cooled prior to introduction into the fluid-energy mill and subsequent to compression and the material to be milled is precooled by the spent carrier gas.

Description

United States Patent 11 1 Weishaupt et a1. July 29, 1975 [54] METHOD OF AND APPARATUS FOR THE 3,614,001 10 1971 Beike 241 23 MILLING OF GRANULAR MATERIALS 3,633,830 H1972 Oberprillezr 241/18 3,658,259 4/1972 Ledergerber 241/5 [75] Inventors: Josef Weishaupt, Pullach; Jakob 3,713,592 1/1973 Beike 241/17 Oberpriller, Baierbrunn, both of G ermany Primary ExaminerGranvi11e Y. Custer, Jr. [73] Assignee: Linde Aktiengesellschaft, Attorney, Agent, or Firm-Karl F. Ross; Herbert Wiesbaden, Germany Dubno 221 Filed: Feb. 7, 1974 [211 App]. No.: 440,446 [57] ABSTRACT Related Application Data In a fluid-energy or jet-milling system in which a gas 0nlinuati0rl 268,134, June 1972, stream entrains a granular or particulate material in a abafldonedjet against a surface to further comminute or pulverize the milled material, the operating or carrier gas is Forelgn Appllcatlon y Data compressed priorito introduction into the mill, ex-

July 2, 1971 Germany 2133019 pands as a jet in the mill chamber and is separated from the comminuted solid matter thereafter. The gas [52] US. Cl. 241/5; 241/18; 241/23 stream is cooled prior to introduction into the fluid- [51] Int. Cl. B02c 19/06 energy mill and subsequent to compression and the [58] Field of Search 241/5, 17, 18, 23 material to be milled is precoolled by the spent carrier gas. [56] References Cited UNITED STATES PATENTS 1 Claim 2 Drawing Flgures 2,836,368 5/1958 McCoy 241/17 METHOD OF AND APPARATUS FOR THE MILLING OF GRANULAR MATERIALS This is a continuation of application Ser. No. 268,134, filed June 30, 1972, now abandoned.

FIELD OF THE INVENTION Our present invention relates to fluid-energy or jet milling and, more particularly, to a method of and an apparatus for the comminution of materials by fluidenergy or jet-milling principles.

BACKGROUND OF THE INVENTION Fluid-energy or jet mills generally make use of a plurality of orifices or nozzles directed toward a wall of a milling chamber or trained on each other and are designed to entrain granular or particulate materials against this wall in a grinding and shearing action to pulverize, abrade and round off the granular materials. Some fluid-energy or jet mills comprise a circular or other round grinding chamber to which the fluid is admitted in fine high-velocity streams at an angle around a portion of, or all of, the periphery of the grinding chamber. In another class of fluid-energy or jet mill, two or more fluid streams convey the particles into a chamber in which the two streams impact upon each other and upon any of the chamber surfaces. Whether the particles are conveyed with the jet or are introduced into the path of jets traversing the grinding chamber, there is a high-energy release and a high order of turbulence which causes the particles to grind upon themselves and to break down by impact, abrasion and friction. Most often, such mills also carry out a classifying operation, i.e., the emerging gas carries off the micronized powder.

In fluid-energy or jet mills it has been proposed to use a driving fluid, i.e., a gas at the grinding temperature,

which is compressed prior to introduction into the jet or fluid-energy mill chamber, which expands in this chamber and which downstream of the chamber is separated from the resulting particles, i.e., micronized powder.

For fine milling and for the finest comminution of granules or coarse particulate substances, such fluidenergy or jet mills have the advantage that no mechanical rotating or oscillating mill parts are required. This not only reduces the cost of the apparatus and the tendency to break down but ensures comminution with a minimum of contamination by foreign matter from the surfaces of a milling chamber.

In counterflow fluid-energy or jet mills, i.e., mills of the character described whereby two particleentraining streams are directed against one another, the carrying gas may be derived from steel bottles or flasks and is permitted to flow through two venturi nozzles in parallel into the fluid-energy or jet mill chamber, the reduced pressure developed in the venturi nozzle together with increased velocity causes the granular or coarse-particle material to be entrained with high speed along a trajectory counter to the trajectory of the particles of the other stream. When two particle streams moving at high velocity and with high kinetic energy in opposite directions collide, the impact releases this kinetic energy in the form of energy of breakdown whereby the structure of the granules is altered or destroyed. Glancing collisions have a similar effect and are also valuable because they provide a mutual abrasion and rounding of the particles.

Fluid-energy or jet milling occurs most readily in the region of the periphery of the milling chamber toward which the particles are carried by the centrifugal forces which induce a rotary movement of the particles within the chamber. The comminuted or subdivided particles which have already been reduced to the desired state of fineness have the smallest mass and tend to collect in the central regions of the milling chamber from which they are removed and may be subjected to further classification in addition to being separated from the expanded gas stream leaving the mill.

The gas/solid separator may be any of the devices known in the art and can include sedimentation chambers, screens, cyclones, and the like. It will be apparent that the kinetic energy of the particles of the counterflow fluid-energy or jet mill appears in large measure as heat which may be detrimental for certain materials so that reduced throughputs may be necessary to prevent such overheating. This solution has the disadvantage, especially in the case of low throughputs, that the energy which is supplied per unit weight or volume of certain materials is excessive while with other materials having relatively significant elasticity and viscosity is insufficient so that satisfactory milling does not occur. In short, fluid-energy and jet milling of synthetic resins and other substances with a high velocity and pronounced plastic deformability cannot be satisfactorily conducted by such apparatus or techniques.

OBJECTS OF THE INVENTION It is an important object of the present invention to provide an improved method of comminuting materials, especially materials of high plastic deformability, especially synthetic resins.

Another object of the invention is to provide an improved method of milling difficult-to-mill materials whereby lower-energy requirements for a given throughput or higher throughputs for a given energy can be attained.

Still another object of the invention is to provide an improved apparatus, and a method of operating same, which is especially effective for materials which have high viscosities and which have not hitherto been adequately or satisfactorily milled by fluid-energy or jetmilling techniques.

SUMMARY OF THE INVENTION These objects and others which will become more readily apparent hereinafter are attained, in accordance with the present invention, with a system which represents an improvement over conventional fluidenergy or jet-milling techniques and involves the cooling of the driving gas or other fluid prior to its expansion in the fluid-energy or jet-milling chamber, but subsequent to its compression. According to a basic principle of the present invention, therefore, a carrier or separating gas is compressed and then cooled, preferably in a heat-exchange relationship with an expanded fluid, prior to introduction into the fluid-energy or jet mill, is then expanded in the chamber of the latter to comminute solid materials entrained by the gas stream or streams, and is separated from the gas subsequent to milling.

An important advantage of the preliminary cooling step, according to the present invention, i.e., the cooling 'of the driving gas prior to its expansion into the fluid-energy or jet mill but after compression, is that it enables the milled material to be brought to low temperatures and even to cryogenic temperatures, i.e., the temperatures of liquefied gases such as nitrogen and oxygen. In this way, the material to be comminuted may be embrittled to facilitate pulverization in the fluid energy or jet mill. In this case, the temperature of the gas is reduced to a point such that the material to be comminuted is no longer plastically or elastically viscous but ruptures readily upon impact with a surface or another particle. In some cases the embrittlement of the material itself may produce a fragmentation. In general, embrittlement of the material to be comminuted results in a substantial increase in the throughput of the apparatus for a given energy and hence represents a substantial increase in efficiency.

According to a more specific feature of the invention, the heat abstracted from the driving gas subsequent to compression and prior to expansion into the mill chamber is transferred by indirect heat exhange to the expanded and thereby cooled gases previously used in the mill chamber, subsequent to separation of the gases from the solids. In other words, the compressed drive-gas stream, before its introduction into the jet mill, is cooled at least in part by heat exchange with the expanded cold drive-gas stream which emerges from the jet mill. The degree of cold retained by the expanded gas stream is, of course, increased as the heat generated in the jet mill is decreased and vice versa. Consequently, any increase in efficiency is multiplied by enabling the low-temperature output gases to cool the incoming compressed gases. Since the cooling step increases efflciency not only by embrittlement and by the removal of large quantities of usable heat from the material subjected to comminution, but for the other reasons mentioned earlier, substantially less heat is produced by conversion of kinetic energy of movement into heat energy of collision or friction so that the maximum cold can be maintained in the expanding gas stream after milling, the mill can be maintained at extremely low temperatures in a convenient manner, and the maximum temperature differential, during heat exchange, between the compressed warm gases and the expanded cold gases is obtained. It should be noted that an increased temperature differential represents an increased heat transfer rate and enables the heat exchanger to be of relatively small dimensions.

A particularly important advantage of the system of the present invention is that with a correct or optimal establishment of the driving-gas quantity, degree of compression, degree of expansion, flow rate, etc., it is possible to obtain embrittlement and the advantages enumerated above with minimum introduction of cold from some foreign source, the energy losses being supplied in the form of power for displacing the fluids. For peak requirements of cold of the drive-gas stream, however, we may make use of an external cold source which may constitute a conventional refrigeration plant or may be a source of liquified gas which can be sprayed directly into the powerfluid lines. The additional cold may also be supplied by indirect heat exchange with a liquified gas. In any case, the external source preferably supplies cold to the drive-gas stream intermittently with the cold pulses having an on-time" and off-time dimensioned to enable the driving fluid to be brought to the lowest suitable temperature and yet enable any established temperature in the milling chamber to be held without difficulty.

According to still another feature of the invention, the driving-gas stream is recirculated in a single path including compression, cooling and expansion, thereby reducing energy and gas consumption and allowing the cooling and milling rate to be controlled with ease. A portion of the cold gas stream may be branched from the main stream and may be passed through the material to be milled before it enters the milling zone, thereby precooling the solids to be comminuted. Advantageously this precooling step is carried out in a column with the solids passing in countercurrent (counterflow) to the branched portion of precooling gas. The precooling can be sufficient to produce the embrittlement mentioned earlier. The partially warm branched gas stream can be then released into the atmosphere or recycled.

Still another feature of the present invention resides in the cooling of the driving-gas stream in a plurality of stages, for example including an initial stage in which it is cooled by heat exchange with the expanded cold gas stream to a low temperature and a second stage in which externally supplied cold brings the gas to still lower temperatures, e.g., in the cryogenic ranges (lowest temperatures). The term cryogenic temperature or cryogenic range is intended to refer to temperatures in a range of the liquefaction temperatures of such gases as argon and the other inert gases, nitrogen and oxygen, or therebelow. These temperatures lie generally below minus C (about 173K). The multistage process enables the energy and cold balance and transfer to be controlled readily and to obtain optimum cooling or embrittlement of the milled product.

As already noted, it is in the nature of the fluidenergy or jet mill that some degree of classification of the comminuted product occurs during milling and in the process whereby the driving gas is separated from the solids. This separation can be enhanced by further classification, e.g., by particle size, or a particular particle-size fraction may be recovered while other fractions are discarded or remain in the mill. Advantageously, water and other solvent vapors released from the milled material and carried off by the expanded gas stream may be removed by absorption in columns or the like prior to recirculation.

When the material to be comminuted is a substance which is not to come into contact with oil, the compression step may be carries out in any dry-running compressor. When oil contamination is not a problem, an oil-lubricated compressor may be used. According to still another feature of the invention, the compression step may take place remote from the installation at which the milling occurs and in that case we prefer to provide steel cylinders or other compressed-gas bottles which may be charged elsewhere with the driving gas. The compressed driving gas is connected via a duct to a heat exchanger, preferably a coiled tube countercurrent heat exchanger, in which the compressed and warm driving gas and the expanded cold gas removed from the chamber are passed in countercurrent. The compressed cooled gas can then be conducted via a further duct to the fluid-energy or jet mill and enters the latter through one or more nozzles.

The operating gas thus expands and subjects the material to be comminuted to the high-energy stream whereby the solid material is pulverized as described conventional dosing device, e.g., a wormor screw adapted to supply the material to be communuted at,

the optimum rate regulated in accordance withthe parameters of the operating gas. The comminuted mate rial, preferably from a shaft or column having the conveyor worm at its base, can be supplied with a branched portion of the cold gas as previously described. From the fluid-energy or jet mill, the expanded cold gas stream, carrying particles of the desired fineness, is delivered to any conventional gas/solid separator in which the milled product is recovered from the expanded operating gas. The gas/solid separator and the heat exchanger are connected by a further duct so that the expanded cold gas stream can be directed to the latter for indirect heat exchange with the compressed warm gas as described. The heat exchanger, in turn, may be connectd with still another duct leading to the compressor so that the expanded gas, after removal of heat from the compressed gas, is supplied to the compressor itself and is passed in a recirculating flow.

The external-cooling device, which may be a refrigerating unit, as noted above, is preferably disposed between the gas/solid separator and the heat exchanger along the cold/gas duct. At least part of the working gases used in the system is preferably conducted through this unit and thereby cooled. In order to obtain an especially high efficiency and reduce the energy supplied to the system, and also to obtain maximum utilization of space, the heat exchanger, material-feed device, material-cooling column, jet mill and separator are so constructed that they form a compact unit which is surrounded by a thermally insulating shell. In this compact configuration, the feed shaft and cooling column are provided above the milling chamber and the gas/solid separator is then located in line therewith so that the gas stream may enter the heat exchanger without entrainment of comminuted material.

DESCRIPTION OF THE DRAWING pact apparatus embodying principles of the invention.

SPECIFIC DESCRIPTION In FIG. 1 of the drawing, we have shown a flow diagram of an apparatus embodying the present invention and which constitutes a cooled jet mill or fluid-energy mill represented generally at 1. Theplant comprises a compressor 2, e.g., a dry-running compressor if the ma terial to be milled must remain freeof oil, or an oillubricated compressor if. oil contamination is no problem. The compressor 2, which may be of the type described at pages 6 2 6 32vof PERRYS CHEMICAL ENGINEERS HANDBOOK (McGraw-Hill Book Co., New York, 1963). preferably should have a capacity such that the emerging gases are at a pressure of about 8 atmospheres gauge. The duct 3a carries the compressed warm gas through a heat exchanger 5 and a heat exchanger 4 supplied with cold water, e.g., from a cooling tower (line 4a), the warm water being reconveyed to the tower via line 4b. The heat exchanger 5 is preferably of the cooled-tube counterflow type wherein the relatively warm compressed fluid passes through the small crosssection channel afforded by the tube coil 5d while the expanded gas stream flows through the large-cross-section shell 5b.

The cooled compressed gas stream is then led via line 3b to the fluid-energy or jet mill 6 which is preferably of the type described at pages 8 42 ff. of PERRYS CHEMICAL ENGINEERS HANDBOOK. The gas expands within the mill chamber and projects a plurality of countermoving jets entraining the particles of material against one another to comminute the material such that the expanded gas stream recovered at 3c entrains the fine particles therewith. The jet mill may be provided with one or more nozzles as described in the cited publication and is preferably of the high-velocity, opposing-jet type.

The material is delivered :from a cooling and feed shaft 7 via a worm conveyor 8 to the mill chamber whereby the particles are comminuted and abraded in accordance with the usual jet-mill principles.

The fine-particle fraction entrained by the gas stream and emerging from the opening 9 of the mill is carried along to the gas/solid separator 10 (line 30) in which the comminuted or pulverized material is separated from the expanded cold gas stream, the particles being collected in a column 10a for further use. The separator 10 may be of any conventional type, e.g., a cyclone or sedimentation column (see pages 20 68 ff. of PER- RYS CHEMICAL ENGINEERS HANDBOOK).

The cold gas, freed from the major portion of the particles in the cyclone and any residual particulate matter in a filter or the like, is passed through an absorption column 15 via line 3d, the absorption column being operated in accordance with conventional principles using calcium chloride or other dessicants. The expanded cold gas is delivered via line 3e to the outer shell of heat exchanger 5 and is passed via line 3f to the compressor 2 to complete the gas cycle represented at Zia-3f.

In the duct 3e which may be provided with a valve 3e so that all or part of the fluid may be passed through level 12a, there is provided a cooling arrangement for delivering external cold to the system or abstracting heat therefrom. The external cooling system comprises the line 12a which conducts all or part of the gas traversing line 3e to a liquefied-gas bath l3, e.g., of liquid nitrogen. The gas of the operating cycle of the mill passes through the tube coil 13a of this tank in which the liquefied gas surrounds the coils and also is provided with a valve 13b whereby some of the cooling fluid is permitted to expand and evaporate into a line 12c. The main body of the diverted portion of the operating gas is carried into line 12b from which it passes via a jet valve 12d into line 32. From line 12c, the evaporated fluid from the external'cooler 13 can be led to line 12b and therebyin'troduced into the cooling cycle.

The external cooler also may be provided with a conduit l4 and a valve 14a leading a portion of the expanded cold gas to the base of the column 7 so that this cold gas passes through the column in counterflow to the descending material to be comminuted. If desired, this cooling stream may be replaced in whole or in part by a portion of the cooled compressed gas which may be bypassed from line 3b via a valve 3b and a duct 3b" to the base of the column 7. The gases used to precool the comminuted material may be vented via a line 14b and a valve 14c or may be returned to line 3f via the conduit 3f. Preferably the valve 3e is pulsed with an on-time and off-time which is determined by a pulser 3e" at a rate such that the external cooling is supplied intermittently, the pulser comprising an oscillator and coil as described on page 13 of PULSE GENERA- TORS by Glasoe and Lebacqz (McGraw-Hill: 1948 The operation of the system of FIG. 1 will be immediately apparent. The compressed gas, which may be at a pressure of 8 atm g. and may be nitrogen, is cooled in the water cooler 4 before entering the heat exchanger 5. In the heat exchanger 5, the compressedgas is cooled to a cryogenic temperature and enters the jet mill 6 in the usual manner. A portion of the gas is diverted through the column 7 and expands therein to precool the material, which may be polyethylene granules, to a low temperature, the embritted granules being carried into the mill 6 and therein comminuted to a particle size of the order of microns. The emerging gas stream, at a cryogenic temperature, is separated from the particles and returned to the heat exchanger in which it cools the compressed gas as described. When the throughput is such that external cooling is required, a portion of the expanded cold gas from the absorber is conducted to the liquid nitrogen tank 13 in which it is cooled to a temperature slightly above the boiling point of nitrogen at atmospheric pressure and is returned to the cold gas stream.

In FIG. 2, we have shown a desirable construction of the mill according to the invention in which the jet mill 16 is surmounted by a feedand cooling shaft or column 17, the latter being surrounded by a heat exchanger shell 18 and provided along its periphery ahead of the heat exchanger with a particle separator 19. The entire structure is surrounded by a thermally insulating layer 20 and stands on a support 21.

The shaft 17 is the upper continuation of the separator l9 and isthe inner tube of the exchanger 18. A chamber 22 is formed within the separator into which the gas from the mill 16 along with the completely comminuted product blows. The separator 19 surrounding this chamber 22 is formed as a screen or grate in order to allow only the gas to pass radially outwardly where it flows over the coils of the exchanger 18. The completely milled particles fall in the chamber 22 into passages 23 which open into a funnel 24 at the base of the installation. Thence these particles pass through a rotary airlock feeder 41 into a container 25.

A stage approximately 4 meters above the base of the device supports a compressor 26 connected in a refrigerant circuit 27 including a reservoir 28 as well as associated equipment 29 (e.g., a compressor as described with respect to FIG. 1). A conduit 30 transfers the compressed gas from the compressor 26 to the coil of the heat exchanger 18 and a conduit 30' conducts the decompressed gas back to the compressor 26.

A conduit 42 leads from the bottom of the heat exchanger coils downwardly in the installation to inject a portion of the compressed gases through a radially opening nozzle 43 at the bottom of the mill 16. The jet pulverizer 16 is of the opposed-jet type. The material to be milled is fed to the pulverizer 16 via a central tube 16a so as to be exposed in the chamber 16b to the opposed jets issuing from nozzles 43 and 44. The particles are milled by striking each other and the sufficiently comminuted granules pass up out of the chamber 16b with the spent milling gas through passages 16c terminating in the chamber 22. Another conduit 44 and nozzle 45 opening directly opposite the nozzle 43 injects gas also into the mill 16 in order to comminute particles in the manner described above.

The refrigerant circuit 27 is connected via a heat exchznger 31 similar to the exchangerS of FIG. 1 to the circuit 30 in order to maintain a sufficiently low temperature in the device. A conduit 32 leads from the path 30 downstream of the exchanger 31 to the base of the precooling and storage shaft 17. Thus the particles in this hopper 17 flow counter-current to the extremely cold gases which are recovered at the top via a conduit 34 and either vented to the atmosphere through a valve 33 or feed back to the circuit 30 through a valve 35 and dust filter 36.

Another rotary airlock feeder 40 is provided at the base of the shaft 17 to prevent gas loss from the mill 16. Yet another such feeder 39 is provided at the top of the shaft 17 so that particles may be fed thereto from a hopper 38 without pressure or material loss.

We claim:

1. A method of comminuting pieces of a material comprising the steps of:

a. confining a supply of said material with a wall and cooling said supply of material by heat exchange with a gas through said wall;

b. compressing said gas after psssing it in heat exchange with said supply of said material through said wall;

c. cooling the gas compressed in step (b) by passing it in heat exchange with a recirculated external coolant to produce a cold fluid;

d. expanding a portion of the cold fluid produced in step (c) and passing the expanded portion of said fluid through said supply in direct contact with said material whereby water and other vapor is entrained with said portion;

e. passing the remainder of the cold fluid produced 1 in step (c) in indirect heat exchange with the gas cooling said supply of material in step (a);

f. metering said material into a jet mill after said material is contacted with said portion of said cold I fluid in step (d);

g. pulverizing said material to a powder in said jet mill by entrainment of said material in the fluid passed in indirect heat exchange with said gas in step (e); f

h. separating the fluid used to pulverize said material instep (g) from the resulting powder; and

i. feeding the fluid separated in step (h) as the gas employed in step (a).

Claims (1)

1. A METHOD OF COMMINUTING PIECES OF A MATERIAL COMPRISING THE STEPS OF: A. CONFINING A SUPPLY OF SAID MATERIAL WITH A WALL AND COOLING SAID SUPPLY OF MATERIAL BY HEAT EXCHANGE WITH A GAS THROUGH SAID WALL, B. COMPRESSING SAID GAS AFTER PASSING IT IN HEAT EXCHANGE WITH SAID SUPPLY OF SAID MATERIAL THROUGH SAID WALL C. COOLING THE GAS COMPRESSED IN STEP (B) BY PASSING IT IN HEAT EXCHANGE WITH A RECIRCULATED EXTERNAL COOLANT TO PRODUCE A COLD FLUID, D. EXPANDING A PORTION OF THE COLD FLUID PRODUCED IN STEP (C) AND PASSING THE EXPANDED PORTION OF SAID FLUID THROUGH SAID SUPPLY IN DIRECT CONTACT WITH SAID MATERIAL WHEREBY WATER AND OTHER VAPOR IS ENTRAINED WITH SAID PORTION, E. PASSING THE REMAINDER OF THE COLD FLUID PRODUCED IN STEP (C) IN INDIRECT HEAT EXCHANGE WITH THE GAS COOLING SAID SUPPLY OF MATERIAL IN STEP (A), F. METERING SAID MATERIAL INTO A JET MILL AFTER SAID MATERIAL IS CONTACTED WITH SAID PORTION OF SAID COLD FLUID IN STEP (D), G. PULVERIZING SAID MATERIAL TO A POWDER IN SAID JET MILL BY ENTRAINMENT OF SAID MATERIAL IN THE FLUID PASSED IN INDIRECT HEAT EXCHANGE WITH SAID GAS IN STEP (E) H. SEPARATING THE FLUID USED TO PULVERIZE SAID MATERIAL IN STEP (G) FROM THE RESULTING POWDER AND I. FEEDING THE FLUID SEPARATED IN STEP (H) AS THE GAS EMPLOYED IN STEP (A).

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